1. Field of the Invention
This invention relates generally to ophthalmological surgery, specifically to a programmable apparatus for modulating a laser beam at high speed for ablating a high resolution corrective profile on a cornea.
2. Prior Art
Myopia (nearsightedness), hyperopia (farsightedness), and astigmatism are refractive errors of the eye that are correctable by reshaping the surface contour of the cornea. A typical procedure involves measuring the shape of a patient's cornea, determining a suitable corrective profile, and reshaping the cornea to match the corrective profile by ablating away a suitably shaped volume of tissue with a spatially manipulated beam from a pulsed laser.
U.S. Pat. No. 4,911,711 to Telfair et al. (1990) and U.S. Pat. No. 5,219,344 to Yoder, Jr. (1993) show laser surgical devices with indexable discs or masks for modulating the cross-sectional shape of an ablating beam. Each mask includes a series of graduated apertures, which are sequentially indexed into alignment with the beam for changing the shape of the ablated area on a cornea. More than one laser pulse may be directed at the cornea through each aperture; the number of pulses fired through each aperture depends on the ablation depth required. Therefore, a desired corrective contour on the cornea is produced by exposing different areas thereon to the beam. However, a large number of different masks must be provided for the large number of possible corrections. Furthermore, the resultant surface contour has a stairstep or terrace shape, wherein the size of the steps are determined by the size difference between adjacent apertures. Because the number of apertures on each mask is physically limited by the diameter thereof, the contours producible are very limited in lateral resolution, i.e., they are rough.
U.S. Pat. No. 5,312,320 to L'Esperance, Jr. (1994) shows a surgical laser device with a partially reflective mirror having varying reflectance along the radius thereof. An input beam having uniform intensity across its diameter is passed through the mirror, which produces an output beam with a precise intensity profile in both the reflected and transmitted components, either of which is usable for ablating the entire surgical zone of a cornea simultaneously. The amount of tissue ablated at any given point is proportional to the laser energy impinging thereon, so that a desired corrective profile can be obtained by using a mirror having a suitable reflectance profile. However, a great number of mirrors having different reflectance profiles are needed for the great number of possible corrections. Furthermore, if a patient requires a correction which does not have a matching mirror, then the closest mirror must be used to produce a less-than-optimal correction.
U.S. Pat. No. 5,281,211 to Parel et al. (1994) shows a surgical laser device with an axicon lens for marking circumferentially arranged spots on a cornea. In another embodiment, masks are used for selectively blocking portions of the beam. This device has the same disadvantages associated with other mask devices.
A further disadvantage of all prior art laser surgical devices with indexable masks is that changing apertures is a slow process, so that a typical operation can take several minutes or more. Such a long surgical procedure is prone to errors, because the eye may shift during the operation.